1. Field of the Invention
This invention relates to the field of transmissions having a layshaft kinematic arrangement, particularly to such transmissions in which gear ratio changes occur without interrupting power flow.
2. Description of the Prior Art
Automatic transmissions for motor vehicle generally employ multiple planetary gearsets whose components are continuously engaged. The gearset components are alternately held and released against rotation by brakes, and are connected and disconnected to components of other gearsets by clutches so that gear ratio changes occur without affecting the continuous gear engagements and without interrupting the flow of power among the gearset components.
Typically manual transmissions have a kinematic arrangement in which a countershaft or layshaft and an output shaft each rotatably support a gear or pinion of a pinion-gear pair associated with a particular gear ratio. Although the gear and pinion are continuously engaged, either the pinion or gear is journalled on its shaft. The journalled component is driveably secured to the shaft by a coupler or synchronizer, which mechanically completes the connection usually after coordinating the speed of the shaft and the speed of the associated gear or pinion. This operation requires use of a friction clutch located in the drive path between the engine output and transmission input. The clutch disengages to interrupt power flow to the transmission before beginning a gear ratio change, and re-engages after completing the gear ratio change to restore power flow to the transmission and drive wheels.
Manual transmissions of this type are characterized by a distinctive, perceptible power flow interruption and the potential for harsh speed ratio changes unless the operator employs care and skill while manually operating a clutch pedal and gear selector in coordination with the engine, driven wheels and driving conditions.
Automatic transmissions for transmitting power between an input and output, either over a continuously variable range of speed ratios or in discrete step changes among speed ratios, have associated with them several sources of parasitic losses, which adversely affect fuel economy. These losses are associated with a torque converter, open clutches and brakes, hydraulic pump, and gear meshes.
In order to avoid these parasitic losses and to improve vehicle fuel economy, an automated shift manual (ASM) transmission has been developed that eliminates many of the low operating efficiency components of an automatic transmission, yet it requires no more driver attention or skill to produce smooth gear ratio changes than does an conventional automatic transmission.
An ASM transmission can eliminate or substantially reduce all of the parasitic losses of an automatic transmission except gear mesh losses. An ASM transmission having a layshaft gear arrangement performs gear ratio changes by first interrupting power or torque transmitted from the engine to the transmission, preparing the transmission components associated with the next speed ratio, and then restoring torque. The need in a conventional ASM transmission to interrupt power transmitted from the engine to the transmission input shaft before each gear ratio change can affect the level of harshness, vibration and noise perceptible to a vehicle occupant.